In so-called single use or disposable writing instruments, the lives of writing instrument tip, adjustment system and content of the writing fluid reservoir are adapted to one another. Prior to using the writing instrument, the writing instrument tip is screwed onto the writing fluid reservoir. A protrusion, which penetrates a diaphragm on the writing fluid reservoir, is located at the threaded end of the writing instrument tip and, as a result of this, flow of the writing fluid to the tip becomes possible. In this respect, reference is made to of German Pat. No. 25 38 427.
As is previously known, a hexagon is provided on the writing instrument tip to enable the writing instrument tip to be screwed into the writing fluid reservoir. A wrench, which has a hexagonal recess, engages with this hexagon. This hexagonal recess can be formed on the cap of the writing instrument or on the writing instrument shaft. Examples of this structure are described in German Design Pat. No. 71 00 878, German Pat. No. 22 22 593 and Japanese Design Pat. No. 58-42132.
The disadvantage associated with these known devices is that the writing instrument tip can be partially or completely unscrewed from the writing fluid reservoir by means of the wrench. If the unscrewing takes place before the writing fluid is completely used up, then contamination, air entry or blockages can occur. When the tip is completely unscrewed, attempts are usually made to reuse an already useless writing tip.